linux-kernel - Re: [RFC PATCH 0/2] arm64: optional paranoid __{get,put}

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Message-ID: <20171101120555.yvb65g3wgtxskfh3@lakrids.cambridge.arm.com>
Date:   Wed, 1 Nov 2017 12:05:55 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Laura Abbott <labbott@...hat.com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        kernel-hardening@...ts.openwall.com,
        Catalin Marinas <catalin.marinas@....com>,
        Kees Cook <keescook@...omium.org>,
        Will Deacon <will.deacon@....com>
Subject: Re: [RFC PATCH 0/2] arm64: optional paranoid __{get,put}_user checks

On Tue, Oct 31, 2017 at 04:56:39PM -0700, Laura Abbott wrote:
> On 10/26/2017 02:09 AM, Mark Rutland wrote:
> > In Prague, Kees mentioned that it would be nice to have a mechanism to
> > catch bad __{get,put}_user uses, such as the recent CVE-2017-5123 [1,2]
> > issue with unsafe_put_user() in waitid().
> > 
> > These patches allow an optional access_ok() check to be dropped in
> > arm64's __{get,put}_user() primitives. These will then BUG() if a bad
> > user pointer is passed (which should only happen in the absence of an
> > earlier access_ok() check).

> Turning on the option fails as soon as we hit userspace. On my buildroot
> based environment I get the help text for ld.so (????) and then a message
> about attempting to kill init. 

Ouch. Thanks for the report, and sorry about this.

The problem is that I evaluate the ptr argument twice in
__{get,put}_user(), and this may have side effects.

e.g. when the ELF loader does things like:

  __put_user((elf_addr_t)p, sp++)

... we increment sp twice, and write to the wrong user address, leaving
sp corrupt.

I have an additional patch [1] to fix this, which is in my
arm64/access-ok branch [2].

Thanks,
Mark.

[1] https://git.kernel.org/pub/scm/linux/kernel/git/mark/linux.git/commit/?h=arm64/access-ok&id=ebb7ff83eb53b8810395d5cf48712a4ae6d678543
[2] https://git.kernel.org/pub/scm/linux/kernel/git/mark/linux.git/log/?h=arm64/access-ok